Conventionally, a Computerized Tomography (hereinafter “CT”) X-ray apparatus is known that irradiate a subject with X-rays, detects X-rays passing through the subject or dispersed by a subject with an X-ray detector and creates either a transparent image, a tomographic image or a three-dimensional image based on the output of the X-ray detector (that is, the amount of X-rays).
A so-called cone beam CT apparatus has been developed as one type of such X-ray CT apparatus. In an ordinary X-ray CT apparatus, the X-ray beam is sliced thinly in a Z-axis direction and is thus called a fan beam, whereas a cone beam CT (CBCT) uses an X-ray beam that extends along the Z axis as well, and for this reason the X-ray beam is called a cone beam.
Moreover, at present, a type of CBCT apparatus corresponding to a so-called third generation or R/R type in the conventional CT apparatus (that is, an apparatus with only 1 row of sensors) is being studied. In a third-generation CT apparatus, a paired assembly consisting of an X-ray source and a detector scan a subject while revolving around the periphery of the subject.
FIG. 9 is a schematic diagram showing an example of the construction of a CBCT apparatus. In the diagram, reference numeral 1 denotes an X-ray tube that serves as an X-ray source, 2 denotes an X-ray detector and 3 denotes a subject. The CBCT apparatus shown in FIG. 9 is a third-generation CT apparatus, and takes the Z axis as its axis of rotation as the X-ray tube 1 together with the X-ray detector 2 move around the subject 3 so as to complete scanning of an area of interest in one rotation.
In an ordinary X-ray CT apparatus, in order to carry out sampling in a channel (CH) direction, sensors are aligned in a single line on the X-ray detector in the CH direction, and each individual sensor element is identified by a channel number. By contrast, in the CBCT apparatus shown in FIG. 9, sensors are arrayed in the CH direction as well as in the Z direction (that is, a ROW direction) on the X-ray detector 2. In other words, the X-ray detector 2 in the CBCT apparatus arranges the sensors in a two-dimensional lattice array crossing at right angles.
According to such a CBCT apparatus, by configuring the detector so that the sensors are arranged in two directions like a lattice, in the Z direction (ROW direction) and the CH direction, and at the same time by emitting radiation thickened in the Z direction (that is, in the shape of a cone), projective data of a plurality of rows can be obtained in a single pass (as is disclosed, for example, by Japanese Patent Application Laid-Open No. 2000-107162).
At the same time, in general radiography, flat panel detectors (FPD) using semiconductors have come to be used. FPDs are also adaptable to CT apparatuses, in which case a cone beam CT apparatus can be constructed.
By contrast, there is also a so-called retrofit CT, in which a FPD and a rotating table are mounted on an ordinary X-ray apparatus, the subject is placed on the rotating table and the subject is sensed while being rotated by the rotating table so as to carry out CT scanning with a three-dimensional X-ray geometric system. Such a system has the advantages of realizing computerized tomography inexpensively, using less space than the conventional CT apparatus (as disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-217810).
In a medical radiographic CT apparatus, the pixel values in the image are calibrated based on water so that portions of the subject that absorb radiation like water have a pixel value of zero and portions corresponding to air have a pixel value of −1,000.
In order to reconstruct the image so that the pixels corresponding to a portion equivalent to water read zero and the pixels corresponding to air read −1,000, a preset phantom is radiographed in advance and water calibration is carried out. However, a CBCT apparatus using an FPD requires a long water phantom extending in the direction of the rotation axis for CT (that is, the Z axis), and such water phantom is very heavy. In addition, radiographing of water phantom for the purposes of water calibration is carried out periodically, and therefore handling a large water phantom can be hard and time-consuming.
In addition, with respect to the retrofit CT, no conventional art discloses technique regarding water calibration.